This invention relates generally to data communication systems and more particularly to local area networks for the transmission of data packets or signals among various transceivers coupled to a transmission medium.
A local area computer network comprises a relatively small data communication system, usually within the confines of several thousand meters, for example, for the interconnection of a plurality of terminal or user devices or hosts, e.g., workstations. The protocol scheme, operational bandwidth and specifications of the network are such that high speed communication among such devices is possible. An overall methodology is to provide hosts, each capable of performing a plurality of computer aided functions independent of any other host coupled to the network, with means to access the network toward the overall goal of intercooperative data communication and improved efficiency in the use of time in conducting business.
A local area computer network generally takes the form of one of three basic topologies, a star, ring or bus. Each such network comprises several basic components. These components include a transmission medium, two or more stations or host and an interface to couple the hosts to the medium. The host makes use of the communication system and is generally a computer. Specialized I/O devices, such as magnetic tape and disk drives, may include sufficient computing resources to function as "hosts" on a network. The transmission medium may be radio, coaxial cable, twisted wire pair or optical fiber. Repeaters may be used along the medium to extend the length of the transmission system beyond the physical contraints imposed by the medium.
The interface includes two basic components, a communication manager, e.g., a controller, and, in most cases, a transceiver. The controller for a host is the set of functions and algorithmic implementation necessary to manage host access to the medium. The basic functions include signal and data handling and transmission management, e.g., signalling conventions, encoding and decoding, serial-to-parallel conversion and vice versa, address recognition, error detection, buffering and packetization. These functions can be grouped into two logically independent sections of the controller: the transmitter and the receiver.
The transceiver contains the electronics to transmit and receive signals to and from the communication medium. A transceiver recognizes the presence of a signal when another host transmits and recognizes a collision that takes place when two or more hosts transmit simultaneously.
An example of a bus oriented local area network is called "Ethernet" and is basically disclosed in U.S. Pat. Nos. 4,063,220 and 4,282,512, assigned to the assignee herein, and is also disclosed in the publication "The Ethernet Local Network: Three Reports" published by Xerox Corporation, Feb., 1980 and in the Ethernet Specifications published Sept. 30, 1980 by Digital Equipment Corporation, Intel Corporation and Xerox Corporation. Ethernet transmission of data signals is by means of packetized data bursts. Each packet contains the identification of the source and destination of that packet, along with a data field generally containing multiple digital data items. Packets are transmitted, for example, along a 50 .OMEGA. coaxial cable at 10 Mbits/s.
The maximum operating distance between Ethernet terminals is about 2,500 meters unless extended by internetwork gateways. A host may be up to 50 meters from the backbone coaxial cable. The maximum number of hosts that may be served is essentially unlimited, providing suitable host interface techniques are employed.
Access to the cable is by contention, using a carrier-sense multiple-access with collision-detection (CSMA/CD) technique. A characteristic of this technique, as in all packet transmission networks, is that access time to the network depends on traffic load.
In Ethernet, as well as in many other local area computer networks, each host has its own transceiver which is coupled by a tap to the communication medium. In many instances, the hosts coupled to a local network are physically located very close together, such as, in the same room or common area. What would be cost effective in such instances is the coupling of several hosts to a single transceiver so that only one transceiver is needed for a group of such "clustered" hosts. In order to accomplish this advantage, some multiplexing scheme must be devised to handle not only the transmission and reception of data signals to and from a group of clustered hosts and their transceiver as well as capable of handling collision detection occurring on the transmission medium but also capable of collision detection among the several client hosts coupled to a single transceiver.